1. Field of the Invention
The present invention relates to a projection lens which projects in an enlarged manner display information, and the like, from a light bulb such as a liquid-crystal display element; and more particularly to a projection lens suitable for use with a display device of rear projection type, such as rear projection TV, and a projection-typed display device using the projection lens.
2. Description of the Related Art
A projection display device of front type and a projection display device of rear type have hitherto been known as a projection-type display device. In the display device of front type, a projection lens is disposed on the same side where a viewer is located with respect to a screen, and light emitted from the projection lens is projected on a reflection-type screen, to thus produce an image on the screen. In the display device of rear type, the projection lens is disposed opposite the viewer with the screen interposed therebetween, and the light emitted the projection lens is projected on a transmissive screen, to thus produce an image on the screen.
Of these rear projection-type display devices, the rear projection display device has been well known to have a structure in which elements ranging from a light source to a screen are housed in a cabinet and in which light carrying video information is projected from a projection lens disposed at the back onto the screen located on the front of the cabinet, as in the case of; for example, a rear projection TV.
Various projection lenses have recently been proposed for application to a projection display device of such a cabinet type (see JP-A-2002-365541, JP-A-2004-177688 and JP-A-2003-57540).
A reduction in profile and thickness of an overall projection display device of cabinet type has been desired. In order to fulfill such a desire, it is necessary to make an entire projection lens system compact by means of a positioning optical path deflection section that deflects an optical path, such as mirrors and a prism, in the projection lens system, to thus lay out the projection lens system in a folded geometry or by means of making the projection lens wide-angle to shorten a projection distance (a distance from the surface of the projection lens closets to the screen to the screen).
A projection display device of a type which forms a color image by use of a plurality of light valves requires a synthesis section for synthesizing light of colors from the respective light valves. Hence, a projection lens having a long back focus has been sought.
Moreover, in conjunction with an increase in the resolution of the light valves, great enhancement of resolution of the projection has been desired. In particular, in the case of a wide-angle projection lens, deterioration of resolution attribution attributable to a chromatic aberration has been taken as a problem.
Since it is difficult to fulfill both such desires, a projection lens fulfilling the desires on a high level have never been realized thus far.
For instance, in JP-A-2002-365541 and JP-A-2004-177688, there is proposed a projection lens which attempts to solve the problem of chromatic aberration by use of three cemented lenses and which has a long back focus. However, the projection lens does not have any space for arrangement of the optical path deflection section, such as a mirror, and hence difficulty is encountered in rendering the entire projection system compact.
In the mean time, a projection lens proposed in JP-A-2003-57540 uses the three cemented lenses and ensures the space. However, compensation of the chromatic aberration arising at the time of use of high-resolution light valves is insufficient. Moreover, since an enlargement-side lens is large, difficulty is encountered in making an attempt to render the entire projection lens system compact.
In view of the above, a projection lens capable of solving these problems is conceived (see JP-A-2007-34082).
Such a projection lens is extremely effective for solving the problems. However, in order to address recent compact light valves, further improvements are sought. Specifically, in these days, the speed of miniaturization of a light valve (a liquid-crystal panel or a DMD) is unprecedented. For instance, the size of one pixel is miniaturized at the rate of 20 to 30 percents or thereabouts, and a display surface of the light valve is also miniaturized significantly. Since the size of an image to be displayed is also miniaturized by far along with miniaturization of the light valves, various contrivances of an optical system, such as shortening of the focal length of the projection lens, are required.